Injection of sufficient Fe 3 ion into mice to saturate serum transferrin promotes a fulminating infection with two otherwise avirulent mutant phenotypes of Yersinia pestis. One of these phenotypes (PGM negative) lacks an ionophore which binds hemin and the other (PST negative) fails to express the bacteriocin pesticin. Using the hydroxamate-specific Csaky assay, the PGM-dependent ionophore will be isolated and characterized with respect to affinity for free and organic iron; a second ionophore of unknown chemical structure will also be so determined. The role of these ionophores in sequestering Fe 3 ion and hemin during infection will be determined with emphasis on defining the consequences of the resulting hemin-deprivation to hepatic microsomes and reticulocytes. These influences will be compared to the effects of iron on neutralizing the antibacterial components of serum, hydrogen peroxide, the lysosomal proteins involved in degranulation and destruction of phagocytized bacteria. The role of pesticin in binding to the PGM-dependent ionophore will be elucidated; this protein will be used as a probe in attempts to solubilize and extract the hemin transport mechanism. The mode of pesticin action as a lysozyme will be determined by sodium dodecyl sulfate gel electrophoresis of peptidoglycan cleavage products. The breakdown of cellular immunity to pesticin in iron-deficient immune serum will be examined and the components necessary for this breakdown will be defined.